A volcano is a hill or
mountain formed when molten material, or lava, from the inside of the
Earth is forced through the Earth’s crust by gases.

A volcano can produce
vast clouds of very fine volcanic dust which looks like smoke.

The lava can become
red-hot, so that at night it looks as if the volcano is on fire. In
this picture we can see the red-hot lava running down the side of the volcano. It is reflected in the cloud of volcanic dust above the volcano, which looks
like a flame.

There are many different
kinds of volcano but they are mostly formed in the same way.

·A hole is blown through the Earth’s crust by the pressure
of gases inside the Earth.

·The hole is widened by further explosions, which scatter
fragments of rock in a circle around the hole.

·These fragments or rock, together with ash, make the
beginning of the cone of the volcano.

·At the top of the cone, the eruption leaves a
crater.

·Lava, the molten material from the inside of the Earth,
wells up through the hole and runs down the sides of the cone.

·If there is no more volcanic activity for a time, the
pipe up the middle of the cone fills with hardened
lava.

·As more gases build up the pressure under the cone, this
hardened lava has to be exploded away in fragments before new outpourings of lava and ash can take
palce.

·The volcanic cone is built up with alternate layers of
lava and ash. When the ash on the side of the mountain has been
pressed down, it is known as tuff.

This photograph shows
the crater at the top of Mount Vesuvius, a famous volcano near Naples in Italy.

Volcanic eruptions are produced by gases, the chief of which is steam. As the eruption dies down, sulphur gases are given off from holes in the floor
of the crater and sometimes from small vents in the side of the volcano. There is one small sulphur vent on the
left of the diagram of a volcano.

Once the activity has
died down further, steam and carbon dioxide escape from the vent.
Later still hot springs of water appear.

Volcanoes are often
found in groups and lines and may be related to mountain-building movements. They are usually found in regions where there have been great rock movements
over millions of years.

Vocanoes can lie
dormant (sleeping) for centuries with no activity at all and then
erupt without warning.

The famous Italian
volcano Vesuvius was known for centuries as a great dome-shaped mountain with a rock-filled hollow on
top. In 72 B.C. some rebellious gladiators hid
there. At that time the hollow was full of wild vines and there
was a lake in the centre. Nobody had ever heard of the mountain
erupting.

One hundred and fifty
one years later, in AD 79, the top of the mountain was blown off and Vesuvius erupted. The red-hot lava and ash buried the cities of Pompeii and Herculaneum.
Thunder, lightning and earthquakes accompanied the eruption.
Pompeii was buried so quickly that everyone was caught and buried at their normal occupations, families sitting
down to a meal, soldiers on guard duty, people carrying their shopping home from market, children in
school.

After this catastrophic
eruption, Vesuvius has had several more eruptions, separated from each other by several hundred
years. There was a serious eruption in 1631 and another in 1906,
which caused great destruction to the surrounding countryside.
There has never been an eruption as serious as the one of 79.

The lava soil around
Vesuvius is rich in minerals and very fertile, allowing farmers to grow wheat and vegetables with little other
feeding of the soil.